Stoichiometric Responses of Soil Microbes and Enzymes to Altitudinal Gradients in Alpine Meadows

Abstract

Soil microbial nutrient limitation is of great significance for the maintenance of soil fertility, the sustainability of plant growth, and the stability of the alpine meadow ecosystems, which are particularly sensitive to global climate change. This study aimed to explore the effects of soil extracellular enzyme activities on soil microbial nutrient limitation across three altitudinal gradients-low altitude (LA: 2900-3200 m above sea level (masl)), middle altitude (MA: 3200-3500 masl), and high altitude (HA: 3500-3800 masl)-in alpine meadows in the northeastern Qinghai-Tibet Plateau, using the method of ecological stoichiometry. The research results showed that soil nutrients mostly accumulate in the surface layer: with increasing altitude, soil organic carbon (SOC) and total nitrogen (TN) contents gradually increase (p < 0.05), and their contents at high altitude in the 0-20 cm soil layer are twice those at low altitude. The activities of β-1,4-glucosidase (BG) and β-1,4-N-acetylglucosaminidase (NAG) at high altitude are significantly 26.77% and 30.88% higher than those at low altitude, respectively. Linear regression analysis shows a significant positive correlation between soil nutrients and C/N/P-related enzymes after logarithmic transformation along the altitudinal gradient. Enzyme vector analysis revealed that in the alpine meadows at altitudes ranging from 2900 to 3800 masl, relative nitrogen limitation was widespread, while relative carbon limitation was more significant in both high-altitude and low-altitude regions (p < 0.05). Notably, this study did not account for the granulometric composition of the soil at the sampling sites. Nevertheless, it partially reveals the nutrient acquisition strategies of microorganisms across different altitudinal gradients, providing a theoretical basis for understanding nutrient cycling in alpine meadow ecosystems and addressing global change.

Keywords: Qinghai–Tibet Plateau; ecological stoichiometry; microorganism nutrient limitation; soil extracellular enzymes.

Figure 1

Location of the study area. LA: low altitude; MA: middle altitude; HA: high altitude.

Figure 2

Soil total nutrients, available nutrients, and microbial biomass under different altitudinal gradients. SOC: Soil Organic Carbon. (a) TN: Total Nitrogen; (b) TP: Total Phosphorus; (c) NO₃⁻-N: Nitrate Nitrogen; (d) NH₄⁺-N: Ammonium Nitrogen; (e) SAP: Soil Available Phosphorus; (f) MBC: Microbial Biomass Carbon; (g) MBN: Microbial Biomass Nitrogen; (h) MBP: Microbial Biomass Phosphorus; (i) different letters indicate values significantly different from each other (p < 0.05) by one-way analysis of variance (ANOVA).

Figure 3

Soil nutrient stoichiometric ratios (SOC:TN (a), SOC:TP (b), TN:TP (c)), microbial biomass stoichiometric ratios (MBC:MBN (d), MBC:MBP (e), MBN:MBP (f)) and extracellular enzyme stoichiometric ratios (Enzyme C:N ratio (g), Enzyme C:P ratio (h), Enzyme C:N ratio (i)) in the 0~20 cm soil layer across different altitudinal gradients. In the figure, different letters indicate values significantly different from each other (p < 0.05) by one-way analysis of variance (ANOVA).

Figure 4

Soil Extracellular enzyme activities of each type and natural logarithmic ratios of enzyme activities between each pair of carbon (C)-, nitrogen (N)-, and phosphorus (P)-related enzymes under different altitudinal gradients. BG: β-1,4-Glucosidase; CBH: β-1,4-Cellobiohydrolase; NAG: β-1,4-N-Acetylglucosaminidase; LAP: Leucine Aminopeptidase; ACP: Acid Phosphatase; E_C:N: Ln(BG + CBH)/Ln(NAG + LAP); E_C:P: Ln(BG + CBH)/Ln(AP); E_N:P: Ln(NAG + LAP)/Ln(AP). In the figure, different letters indicate values significantly different from each other (p < 0.05) by one-way analysis of variance (ANOVA).

Figure 5

Ln-transformed stoichiometric relationship diagrams between soil nutrient elements, soil microbial biomass, and C-N-P-related enzymes, as well as between soil microbial biomass and its resources. The figure includes: relationships of carbon homeostasis (a), nitrogen homeostasis (b), and phosphorus homeostasis (c) across different altitudinal gradients; and relationships of carbon-nitrogen homeostasis (d), carbon-phosphorus homeostasis (e), and nitrogen-phosphorus homeostasis (f) across different altitudinal gradients. Abbreviations: R_C:N, R_C:P, and R_N:P represent the carbon-to-nitrogen ratio, carbon-to-phosphorus ratio, and nitrogen-to-phosphorus ratio of total soil nutrients, respectively; B_C:N, B_C:P, and B_N:P correspond to the carbon-to-nitrogen ratio, carbon-to-phosphorus ratio, and nitrogen-to-phosphorus ratio of soil microbial biomass, respectively.

Figure 6

Relationship diagram between different altitudinal gradient treatments, vector length (a), and vector angle (b) derived from vector analysis of soil extracellular enzymes. In the figure, different letters indicate values significantly different from each other (p < 0.05) by one-way analysis of variance (ANOVA).

Figure 7

Results of standardized major axis (SMA) regression (Type II) analysis. The figure presents: (a) the relationship between carbon (C)-acquiring enzyme activity (represented by the sum of β-1,4-glucosidase (BG) and β-1,4-cellobiohydrolase (CBH)) and nitrogen (N)-acquiring enzyme activity (represented by the sum of β-1,4-N-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP)); (b) the relationship between C-acquiring enzyme activity (BG + CBH) and phosphorus (P)-acquiring enzyme activity (represented by acid phosphatase (ACP)); (c) the relationship between N-acquiring enzyme activity (NAG + LAP) and P-acquiring enzyme activity (ACP); (d) the relationship between β-1,4-glucosidase (BG) activity and nitrogen (N)-acquiring enzyme activity (represented by the sum of β-1,4-N-acetylglucosaminidase (NAG) and leucine aminopeptidase (LAP)); (e) the relationship between β-1,4-glucosidase (BG) activity and phosphorus (P)-acquiring enzyme activity (acid phosphatase (ACP)); (f) the relationship between β-1,4-cellobiohydrolase (CBH) activity and N-acquiring enzyme activity (represented by the sum of NAG and LAP); (g) the relationship between β-1,4-cellobiohydrolase (CBH) activity and P-acquiring enzyme activity (acid phosphatase (ACP)). All data were subjected to logarithmic transformation.

Figure 8

Results of the final piecewise structural equation model (SEM). The model illustrates the direct effects of the altitudinal gradient on soil nutrients and extracellular enzymes, as well as the indirect effects thereby induced among soil extracellular enzyme stoichiometry, microbial biomass stoichiometry, and soil nutrient stoichiometry. The path coefficients (correlation coefficients) beside the arrows were standardized using the mean values of each parameter. Green arrows and red arrows represent significant positive correlations and significant negative correlations, respectively (p < 0.05), while dashed lines indicate non-significant relationships (p > 0.05). The percentages labeled next to the variables represent the variance explained by the model (R²). Symbols *, **, and *** indicate statistical significance at the levels of p < 0.05, p < 0.01, and p < 0.001, respectively.